President Christina Fernández de Kirchner met Yair Tawil and his family at her office last week to mark the unusual ceremony, which dates back more than 100 years.

According to Argentinian folklore, the seventh son born to a family turns into the feared “el lobison”.

The werewolf-like creature shows its true nature on the first Friday after boy’s 13th birthday, the legend says, turning the boy into a demon at midnight during every full moon, doomed to hunt and kill before returning to human form.

As well as feeding on excrement, unbaptized babies, and the flesh of the recently dead, the lobison was said to be unnaturally strong and able to spread its curse with a bite.

Fear of the lobison was so rife in 19th Century Argentina that some families abandoned or even murdered baby boys – an atrocity that sparked the unusual Presidential practice of adoption, aimed at stopping the deadly stigma.

Starting in 1907, it the tradition was formally established by a decree in 1973 by Juan Domingo Peron, which also extended the practice to baby girls.

Seventh sons or daughters – now much rarer than 100 years ago – gain the President as their official godparent as well as a gold medal and full educational scholarship.

Even now, reports of dog-like creatures attacking livestock continue, as does the tradition.

Ms Fernandez said Yair is the first Jewish boy to be adopted, as the tradition only applied to Catholic children until 2009.

She described her meeting with him and his family on 23 December as a “magical moment”.

Calling the Tawils a “marvellous family” she described Yair as “a total sweety” and dubbed his mother “Queen Esther.”

Shlomo and Nehama Tawil, parents of seven boys, had written the President a letter in 1993 and got their wish this year, the Jewish Telegraphic Agency reported, attending the ceremony with their son and three of his brothers.

Pictures showed them lighting Hanukkah candles together on a menorah from Israel presented to the president by the Tawil family.